Methods to enhance cell migration and engraftment

ABSTRACT

A method is provided to functionally select cells with enhanced characteristics relevant to cell engraftment, including both spontaneous migration and directional migration towards specific chemo-attractants. The cells are preferably undifferentiated cells, such as mesenchymal stem cells. The method involves entrapping or encapsulating the cells in a biomaterial barrier, optionally inducing cell migration, and selecting cells that migrated through the barrier. The cells selected by this method have better migratory activities and enhanced in vivo engraftment to injured tissues when they are supplemented systemically.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 11/750,863, filed May 18, 2007, which claimsbenefit of and priority to U.S. Provisional Patent Application No.60/801,975 filed May 19, 2006. This application claims benefit of U.S.Provisional Patent Application No. 61/354,871, filed Jun. 15, 2010. Allof the above are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention is generally related to the field of cell engraftment,more particularly to methods and compositions for enhancing bothspontaneous migration and directional migration of transplanted cellpopulations.

BACKGROUND OF THE INVENTION

Cell-based therapy, particularly using undifferentiated cells, e.g.,multipotent or pluripotent cells stem cells, presents a promisingapproach for regenerative medicine and tissue engineering (Short et al.Arch Med. Res. 2003 34(6):565-71, Barrilleaux et al. Tissue Eng. 200612(11):3007-19). In particular, bone marrow mesenchymal stem cells(MSCs), or stromal stem cells, have been shown to be beneficial inregenerating tissues of musculoskeletal (Horwitz et al. Nat. Med. 1999March; 5(3):309-13), cardiovascular (Pittenger et al. Circ Res. 200495(1):9-20; Chen et al. Chin Med J (Engl). 2004 117(10):1443-8. Erratumin: Chin Med J (Engl). 2005 118(1):88; Price et al. Int J Cardiol. 2006111(2):231-9; Wang et al. Crit. Care Med. 2007 35(11):2587-93) andneurological (Helm et al. Neurosurg Focus. 2005 15; 19(6):E13) systems.This is because of their relatively safety and easy accessibility fromthe donor, genetic stability, and the ability of self-renewal anddifferentiating into multiple lineages of cells (Pittenger et al.Science 1999 2; 284(5411):143-7; Pittenger et al. Circ Res. 200495(1):9-20). They also have the unique immunologically privileged statuswhich can ignore the Human Leukocyte Antigen histocompatibility barrier(Pittenger et al. Circ Res. 2004 95(1):9-20). It is also more sociallyand ethically acceptable to use MSCs than embryonic stem cells.

Despite the great potential of MSC-based therapies, the functionaloutcomes of existing MSC therapies have not been satisfactory, at leastpartly due to the extremely poor engraftment rate (usually <1-2%) ofMSCs at the target tissues such as hearts in the treatment of myocardialinfarction (Price et al. Int J. Cardiol. 2006 111(2):231-9; Wang et al.Crit. Care Med. 2007 35(11):2587-93) and bones in the treatment ofosteogenic imperfecta (Horwitz et al. Nat. Med. 1999 5(3):309-13) uponsystemic injection. It is generally believed that the outcomes of MSCtherapies can be further enhanced if the engraftment efficiency of humanMSCs can be improved. As a result, strategies aimed to enhance theengraftment rate are important to improving the efficacy of MSCs-basedtherapies.

Local injection of MSCs with and without carriers is able to improveengraftment in the target tissue such as heart and intervertebral disc,but the extent of improvement is less than 5%, which is too low togenerate a significant impact. Moreover, the invasiveness associatedwith the open surgery-based injection into inner organs such as heartand spine, and the inapplicability of local injection in diseases wheremultiple tissues are involved, such as osteogenic imperfecta and muscledystrophy, prevents this approach from being widely used in regenerativemedicine. Currently, no effective method exists to improve theengraftment rate of systemically supplemented MSCs.

Stem cell engraftment, sometimes used interchangeably with stem cellhoming, describes the process of directing undifferentiated cells, suchas MSCs, to migrate from the peripheral blood into the damaged tissues(Chamberlain et al. Stem Cells. 2007 25(11):2739-49; Chavakis et al. JMol Cell Cardiol. 2008 45(4):514-22). The detailed mechanisms of stemcell engraftment are still unclear but directional movement of stemcells towards gradients of chemoattractants and cytokines induced bytissue injuries is known to play an important role (Ponte et al. StemCells. 2007 25(7):1737-45; Sordi et al. Blood. 2005 106(2):419-27; Sonet al. Stem Cells. 2006 24(5):1254-64; Honczarenko et al. Stem Cells.2006 24(4):1030-41; Ries et al. Blood. 2007 109(9):4055-63). Among allchemoattractant and receptor pairs, SDF-1 and its specific receptorCXCR4 have been shown to be most important in regulating the migrationof hMSCs [Ponte et al. Stem Cells. 2007 25(7):1737-45; Dar et al. Nat.Immunol. 2005 6(10):1038-4; Dar et al. Exp Hematol. 2006 34(8):967-75;Honczarenko et al. Stem Cells. 2006 24(4):1030-41).

Several attempts have been made recently to enhance the responsivenessof MSCs towards specific chemoattractants. In particular, MSCs have beentreated with chemoattractants such as SDF-1 (Shi et al. Haematologica.2007 July; 92(7):897-904; Ponte et al. Stem Cells. 2007 25(7):1737-45),or virally transduced with chemoattractant receptor genes such as CXCR4(Bhakta et al. Cardiovasc Revasc Med. 2006 7(1):19-24). Both methodsupregulate the expression of the chemoattractant receptor on MSCs. A fewother approaches include viral transduction of MSCs with anti-apoptoticgenes such as Akt and telomerase to promote MSC survival (Seeger et al.Nat Clin Pract Cardiovasc Med. 2007 4 Suppl 1:S110-3) and extrinsicsupplementation of chemoattractants such as SDF-1 at the injured targettissues to augment the chemoattractant signal for MSC recruitment at theinjury site (Yamaguchi et al. Circulation. 2003 107(9):1322-8).Nevertheless, these approaches also have significant drawbacks such asthe long term uncertainty of genetically manipulated cells, the safetyissue of virally transduced cells and the poor cost-effectiveness of invivo supplementation of chemoattractants. More importantly, none ofthese attempts address the heterogeneity problem of MSCs.

It is therefore an object of the invention to provide methods andcompositions to promote or enhance migration, engraftment, or acombination thereof of transplanted cells, such as MSCs.

SUMMARY OF THE INVENTION

A method to process undifferentiated cells for engraftment, such asMSCs, by functionally selecting subpopulations of undifferentiated cellswith better migratory activity has been developed. The method enhancesboth spontaneous migration and directional migration of the cellstowards specific chemo-attractants. The method also provides a means toenhance the in vivo engraftment rate of undifferentiated cells duringtissue repair and regeneration.

In preferred embodiments, the method involves entrapping orencapsulating undifferentiated cells in a biomaterial barrier,optionally inducing the cells to migrate, and selecting cells thatmigrate through the barrier. The selected cells have bettercharacteristics such as in vitro migratory activities and in vivoengraftment rate to injured tissues when they are supplementedsystemically. The cells can be undifferentiated cells, such asmultipotent or pluripotent stem cells. In preferred embodiments, thecells are mesenchymal stem cells (MSCs), such as human MSCs (hMSCs).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating the disclosed method of selectingsubpopulations of undifferentiated cells with better migratory activity.

FIGS. 2A and 2B are line graphs showing the dose-dependent (ng/ml)migratory response of 2D cultured hMSCs (number migrated cells) as afunction of chemokine concentration (ng/ml) of Fractalkine (FIG. 2A) andSDF-1β (FIG. 2B).

FIGS. 3A, 3B, and 3C are bar graphs showing the relative number(normalized to the number of cells migrating in 2D culture) of 2Dcultured hMSCs, 3D cultured hMSCs (cultured in collagen microspheres atfree-floating conditions), 3D remained hMSCs (those remaining inmicrospheres after culture on substratum), 3D migrated hMSCs (thosemigrating out of microspheres cultured on substratum), and subculturedsubpopulations of hMSCs cells from different donors (subject 1 andsubject 2) that were migrating spontaneously (no chemoattractant) (FIG.3A, control), migration towards Fractalkine (FIG. 3B, 10 ng/ml), andmigration towards SDF-1 (FIG. 3C, 50 ng/ml).

FIG. 4 is a bar graph showing the relative Transwell migratory activityof the 3D migrated hMSC subpopulation from collagen barriers ofdifferent collagen concentrations (0, 0.5, 1, and 2 mg/ml) normalized tothe Transwell migratory activity of the 2D cultured hMSCs.

FIG. 5 is a bar graph showing the in vivo engraftment rate (% of livercells positive for human HLA-ABC) of 3D migrated hMSC subpopulation, 2Dcultured hMSCs, and 1×PBS control in partial hepatectomized NOD/SCIDmice.

FIGS. 6A, 6B, and 6C are bar graphs showing the relative number(normalized to the number of cells migrating in 2D culture) of 2Dcultured hMSCs, 3D cultured hMSCs (cultured in collagen microspheres atfree-floating conditions), 3D remained hMSCs (those remaining inmicrospheres after culture on substratum), 3D migrated hMSCs (thosemigrating out of microspheres cultured on substratum), and subculturedsubpopulations of hMSCs cells from derived from adipose tissue that weremigrating spontaneously (no chemoattractant) (FIG. 6A, control),migration towards SDF-1 (FIG. 6B, 50 ng/ml), and migration towardsFractalkine (FIG. 6C, 10 ng/ml).

FIG. 7 is a bar graph showing the relative number (normalized to thenumber of cells migrating in 2D culture) of 3D migrated hMSCsencapsulated in 0.5 mg/ml collagen cultured at a density of 250 cell/μl,750 cell/μl, or 1250 cell/μl that were migrating spontaneously (nochemoattractant, open bars), migration towards SDF-1 (50 ng/ml, shadedbars), and migration towards Fractalkine (10 ng/ml, solid bars).

FIG. 8 is a bar graph showing the relative amounts of matrixmetalloprotease 1 (MMP1) (normalized to total protein) secreted by 3Dmigrated hMSCs (those migrating out of microspheres cultured onsubstratum), 3D remained hMSCs (those remaining in microspheres afterculture on substratum), 2D cultured hMSCs, and 3D cultured hMSCs(cultured in collagen microspheres at free-floating conditions).

FIG. 9 is a bar graph showing the relative number (normalized to thenumber of cells migrating in 2D culture) of 2D cultured hMSCs and 3Dmigrated hMSCs adhering to endothelial cells.

DETAILED DESCRIPTION OF THE INVENTION

A common challenge in cell-based therapies using undifferentiated stemcells, such as human mesenchymal stem cell (hMSC), is limitedengraftment efficiency. One of the key steps governing the engraftmentrate is the ability of stem cells, including but not limited topluripotent or multipotent cells, to migrate to the targeted injurysites. As a result, strategies for enhancing the migratory activitiesare important for development cell engraftment therapies.Undifferentiated cell populations, such as hMSCs, generally containheterogeneous mixtures of multiple cell types, which differ inmorphology, phenotype, and functional properties including migratoryactivities. A method of selecting undifferentiated stem cells, such ashMSCs, with better migratory activities using a biomaterial barrier,such as collagen, is described.

In one embodiment, hMSCs were subjected to a self-selection process viamicroencapsulation in a collagen barrier and induced to migrate out fromthis barrier. The hMSC subpopulation that migrates out of the barrierhas a significantly better migratory response, both spontaneouslytowards serum free medium and directionally towards well-knownchemoattractants, as compared to other subpopulations, including thoseremaining inside the collagen barrier and those in traditional 2Dcultures. Moreover, the selection of hMSCs by this method is positivelyassociated with the concentration of the collagen barrier and the celldensity.

I. DEFINITIONS

As used herein, “cell engraftment” or “cell homing” refers to theprocess by which cells, such as stem cells, that are transplanted into asubject incorporate into tissues of the subject.

As used herein, “crude preparation of cells” refers to a heterogeneouspopulation of cells isolated from a tissue source prior to any selectionprocess.

As used herein, “inducing cells to migrate” refers to the use ofconditions that are suitable for, or promote cell migration. Theconditions include physical, biological, and chemical stimuli thatpromote migration of cells, including, but not limited to, culture on asubstratum and the use of serum or specific chemokines in the culturemedium.

As used herein, “spontaneous migration” refers to the migration of cellsin the absence of specific chemoattractants.

As used herein, “directional migration” refers to the migration of cellstowards chemoattractants.

As used herein, “stem cell” refers generally to an undifferentiated cellregardless of source, and includes multipotent cells or pluripotentcells. Stem cells include de-differentiated cells, embryonic stem cells,mesenchymal stem cells, and induced pluripotent stem cells. Stem cellscan be embryonic or adult stem cells.

As used herein, “progenitor cell” refers generally to unipotent oroligopotent cells that do not replicate indefinitely.

As used herein, “totipotency” refers to a single undifferentiated cellwith the ability to divide and produce all the differentiated cells inan organism, including extraembryonic tissues.

As used herein, “pluripotency” refers to a single undifferentiated cellwith the ability to differentiate into cells of any of the three germlayers: endoderm (e.g., interior stomach lining, gastrointestinal tract,the lungs), mesoderm (e.g., muscle, bone, blood, urogenital), orectoderm (e.g., epidermal tissues and nervous system). Pluripotent cellscannot develop into a fetal or adult animal because they lack thepotential to contribute to extraembryonic tissue, such as the placenta.

As used herein, “multipotent” refers to a single undifferentiated cellwith the ability to differentiate into multiple cell lineages but not tocells of all three germ layers.

As used herein, “oligopotent” refers to a single undifferentiated cellwith the ability to differentiate into a few cell types.

As used herein, “subject” refers to any individual who is the target ofadministration. The subject can be a vertebrate, for example, a mammal.Thus, the subject can be a human. The term does not denote a particularage or sex. A patient refers to a subject afflicted with a disease ordisorder. The term “patient” includes human and veterinary subjects.

II. METHODS OF MODIFYING MSC MIGRATION

A representative method for modifying stem cell migration includesfunctionally selecting cells with enhanced characteristics relevant tocell engraftment, such as migratory activities. The method involvesentrapping cells, such as a crude cell pellet, in a biomaterial barrieror a gradient of one or more biomaterial barriers. The cells areobtained from conventional processing methods such as adhesion selectionor flow cytometry sorting. Cells for use in the selection method can beundifferentiated or mature differentiated cells that are suitable forcell engraftment. Preferred cells are undifferentiated stem cells orprogenitor cells. The specific source of undifferentiated cells can beselected based on the target tissue for engraftment. For example,mesenchymal stem cells (MSCs) are particularly suitable formusculoskeletal, cardiovascular and neurological systems. MSCs can beobtained from various sources, including, but not limited to, bonemarrow, adipose tissue, umbilical cord, and blood. In addition, MSCs canbe derived from pluripotent stem cells, such as induced pluripotent stem(iPS) cells and embryonic stem (ES) cells.

The biomaterial barrier used should be able to entrap or encapsulate thecells under physiologically relevant conditions yet allow the entrappedcells to penetrate or invade and then migrate through or transmigrateunder certain conditions. As used herein, “functional” means the abilityto migrate through a barrier. As a result, many biomaterials can beused, including but are not limited to extracellular matrix materials(“ECM”) such as collagen, fibrin, Matrigel™, self-assembled peptides,and hyaluronic acid. Collagen is a preferred example. Moreover, thebarrier can be in any form, including, but not limited to, microspheres,block gel, cylindrical shaped, patch, and thin film.

The barrier can be a homogenous barrier with a homogeneous fiber densityor barrier capacity, or a gradient barrier of increasing fiber densityor barrier capacity, or any combination with other material barrierselectively allowing migration of different cells. Numerous fabricationtechnologies such as reconstitution, self-assembly, nestedself-assembly, 3D printing, photopolymerization, and electrospinning canbe used to form the barrier.

The following is an exemplary description for selecting hMSCs for cellengraftment using collagen microencapsulation. It is understood that thefollowing steps can be adapted to other cells and biomaterials.

A. Trapping Crude hMSCs in a Biomaterial Barrier

A microencapsulation technique is used to entrap hMSCs within abiomaterial barrier in a three dimensional configuration. Crude hMSCs(referred to as two dimensional (2D) cultured) are obtained from varioussources including, but not limited to, bone marrow, cord blood, andplacenta. The hMSCs are obtained at a concentration of 1×10³ cells/ml to1×10⁶ cells/ml, preferably 1×10⁴ cells/ml.

In some embodiments, the hMSCs are cultured 3 days to 14 days prior tomicroencapsulation. For example, the hMSCs can be cultured in a fullmedium containing Dulbecco's modified Eagle's medium-low glucose(DMEM-LG), 10% fetal bovine serum (FBS), 100 U/ml penicillin, 100 mg/mlstreptomycin and 1% glutamax, or full medium with lower % of FBS such as5%, or other medium able to support the growth of MSCs. The hMSCs arepreferably cultured from about 50% to about 99% confluence, morepreferably 80% confluence. For example, the hMSCs can be cultured forabout 3 days to about 14 days, including about 6 days. Once the hMSCsreach the desired confluence, the cells are enzyme digested (e.g.,trypsinized) and suspended for subsequent microencapsulation.

In preferred embodiments, the biomaterial barrier is collagen, which ismixed with the hMSC suspension. For example, rat-tail collagen type 1solution can be neutralized by 1N NaOH and mixed with the hMSCsuspension. The final collagen concentration is preferably about 0.5mg/ml and the final cell density is about 1×10³ cells/ml to 1×10⁷cells/ml, preferably about 1×10⁵ cells/ml.

The cell-collagen mixture is dispensed as structures, including but notlimited to, patches, droplets, thin layers, and blocks, onto acollection platform with a non-adherent surface. The structures areincubated at 37° C. for at least 1 hour to allow for reconstitution intosolid structures, which is then collected into a container, such as aPetri dish, supplemented with full medium.

B. Induction of hMSC Migration Through the Barrier

Since hMSCs are adherent cells, they prefer a substratum for attachment.By providing a substratum, such as a rigid substratum of culture dish orsoft collagen gel matrix, to the MSC-encapsulated collagen barrier, somehMSCs are able to migrate out through the collagen fibrous meshwork withdifferent densities or concentrations. The method can also include astep of allowing interactions of the entrapped cells with thebiomaterial barrier for certain period of time in suspension or freefloating cultures prior to culture on a substratum. The duration of thisculture period ranges from 0 hours to 14 days, preferably 3 to 4 days.This is to allow the cells to adapt to the 3D matrix environment and thesoft matrix barrier of the microspheres to contract to an equilibriumstatus.

These encapsulated cells are then transferred into a tissue culture dishwith an adherent surface or a collagen gel cushion. A minimal amount ofculture medium can be supplemented to prevent “free-floating” of thestructures after attaching to the substratum of the tissue culture dishfor a period of time sufficient for the structures to attach to thesubstratum provided, ranging from 0.5 hour to 10 hours, preferably 0.5hour. Full medium is carefully supplemented afterwards so as not tomechanically disturb the attached structures. After about three days,hMSCs that have migrated out from these structures to the adherentsubstratum of the tissue culture dish are collected. In someembodiments, these collected cells are functionally selected hMSCs. Inother embodiments, the collected cells are cultured for another periodof time for expansion, ranging from one day to 14 days, preferably threedays, then collected as functionally selected hMSCs.

Optionally, the barriers are detached from the substratum by mechanicalmeans such as agitation or flushing with medium. The detached barrierscan be plated again to start another round of inducing the entrappedcells to migrate out.

The functionally selected hMSCs can also be enriched by methods such assubcultures or clonal selection, before use in cell engraftment.

A kit containing materials, tools and protocols for the functionalselection of MSC subpopulations with better migratory activities andtherefore engraftment can be prepared for use in the method describedabove, and as demonstrated in the following examples. The associatedproduct can be a kit for cell processing. The kit includes two or moreof a device for collecting the tissue source, a device to do initialselection of MSCs, devices and reagents to do the encapsulation and theinduction steps, devices to enable migration selection, devices andreagents for collection of the functionally selected cellsubpopulations, and devices for systemic injection or implantation ordelivery of the processed cells. The components of the kit are packagedin a container, typically a container suitable for shipping.

III. SELECTED MSC POPULATIONS

MSCs are present in extremely low percentages (<0.1%) in bone marrow andother sources such as adipose tissues. They can be separated fromhaematopoietic stem cells and other cells by negative immunoselectionfor haematopoietic and endothelial markers such as CD34 and CD45(Deschaseaux et al. Br J Haematol. 2003 122(3):506-17), positiveimmunoselection for Stro-1 (Simmons et al. Blood. 1991 Jul. 1;78(1):55-62), or adhesion selection based on their ability to adhere tothe culture substratum (Tondreau et al. Cytotherapy. 2004 6(4):372-9;Pittenger et al. Circ Res. 2004 95(1):9-20). Nevertheless, MSCs isolatedby these methods are still a heterogeneous mixture of multiple types,which differ in morphology, phenotype, genotype and functionalproperties including migratory activities (Colter et al. Proc Natl AcadSci USA. 2001 98(14):7841-5; Sordi et al. Blood. 2005 106(2):419-27;Digirolamo et al. Br J Haematol. 1999 107(2):275-81). Therefore,transplanting MSC as a crude mixture results in low engraftment.

The cells obtained using the method described above are enrichedsubpopulations of MSCs with better intrinsic migratory activities bothspontaneously and directionally towards chemokines secreted into thecirculation during tissue injuries, such as Fractalkine and SDF-1. Moreimportantly, the selected MSC subpopulations have a better engraftmentrate to the injured tissue such as liver, heart, and bone, when injectedsystemically. This may be due to better migratory activities alone, orin combination with other activities such as better transmigrationthrough the endothelial barrier, better survival at the hostileenvironment of the tissue defect, and better survival and functionalremodeling at the defect site. The functionally selected stem cells haveunaltered, if not improved, self-renewal capacity and multipledifferentiating potential.

IV. METHODS OF USE

Cell populations, e.g., hMSCs, functionally selected by the disclosedmethod are useful in treating tissue injuries and in regenerativemedicine and tissue engineering. For example, in cell based regenerativemedicine for tissue injuries, the functionally selected cells can beinjected to the blood stream of individuals with defective tissues.Functionally selected cells, such as hMSCs, have enhanced engraftment intissue defects and therefore improve the functional outcome ofcell-based therapy.

The present invention will be further understood by reference to thefollowing non-limiting examples.

EXAMPLES Example 1 Microencapsulation of hMSCs Using Collagen Barrier

Materials and Methods

Bone marrow aspirates were collected from two healthy bone marrow donors(Subject 1 and 2) with informed consents. hMSCs were cultured in fullmedium consisting of Dulbecco's modified Eagle's medium-low glucose(DMEM-LG), 10% fetal bovine serum (FBS), 100 U/ml penicillin, 100 mg/mlstreptomycin and 1% GlutaMax™ at 37° C. with 5% CO₂. Cells at passage 4were subcultured as traditional 2D (monolayer) cultures. The initialcell seeding density of traditional 2D culture was 6.25×10⁴ per 100 mmtissue culture dish.

After trypsinization, cells at passage 5 were labeled “2D culturedcells.” Some of these cells were used for subsequent microencapsulation.hMSCs were trypsinized using 0.05% Trypsin-EDTA (Gibco).

Cells were microencapsulated in a collagen barrier as described by Chan,et al. Biomaterials 28 (2007) 4652-4666. Rat-tail collagen type 1solution (BD Biosciences) was neutralized by 1N NaOH and diluted to afinal concentration of 0.5, 1, 2 or 3 mg/ml in the presence of hMSCs inDMEM-LG. The final cell density was 1×10⁵ cells/ml or 5×10⁵ cells/ml.The cell-collagen mixture was dispensed as 2.5 μl droplets onto acollection platform with non-adherent surface. The microdroplets wereincubated at 37° C. for 1 hour to allow for reconstitution into solidmicrospheres, which were then collected into 90 mm Petri dishsupplemented with full medium.

Two hundred and fifty microspheres were collected into each Petri dishsuch that the total cell number of each Petri dish was equivalent to theinitial cell seeding density of traditional 2D culture for comparison.Images of samples under optical microscope (Leica) were taken atdifferent time points to evaluate the size and morphology of thehMSCs-collagen microspheres. The hMSCs-collagen microspheres werecultured at free-floating condition in a 90 mm Petri dish supplementedwith full medium for 3 days. Some of the microspheres were then digestedusing collagenase (200 unit/ml; Sigma) followed by 0.05% Trypsin-EDTA tofurther separate the cells into single cell suspension. The cellscollected were labeled as “3D cultured cells.”

The other microspheres were transferred from Petri dish into tissueculture dish and were attached to the substratum of the tissue culturedish supplemented with full medium for 3 days to allow the hMSCsentrapped inside the collagen matrix to migrate out to the surroundingsubstratum. After that, the microspheres were detached from thesubstratum and digested using collagenase followed by 0.05% Trypsin-EDTAto collect cells remaining within the microspheres. These cells werelabeled as “3D remained cells.”

Cells that migrated out from the collagen microspheres were furthercultured for 3 days, trypsinized using 0.05% Trypsin-EDTA and labeled as“3D migrated cells.”

Some of the 3D migrated cells were further subcultured in full mediumand the cells collected afterwards were labeled as “Subcultured cells.”

Results

FIG. 1 shows the overview of all treatments and labeling of differentcell subpopulations. hMSCs were entrapped in a dense collagen fibermeshwork barrier. Cells were entrapped but not migrated out whencultured free floating or in suspension, while some cells migrated outwhen the barriers were attached or plated to a substratum. The 3Dmigrated cells had a more homogenously smaller cell morphology ascompared to the 2D cultured cells.

Example 2 Transwell Migratory Activities of Different hMSCSubpopulations

Materials and Methods

Serum free medium alone or in the presence of either 10 ng/mlFractalkine (CX₃CL1; Peprotech) or 50 ng/ml SDF-10 (CXCL12; Peprotech)in a total volume of 800 μl was added into the lower chamber of a24-well plate transwell (BD Biosciences). A cell culture insert 8 μmpore size (BD Falcon™ Cell Culture Inserts, catalog #353097) was gentlyplaced into the well. An aliquot of 5×10⁴ hMSCs collected from thedifferent treatment groups of Example 1: 2D cultured cells, 3D culturedcells, 3D remained cells, 3D migrated cells or Subcultured cells, wassuspended in 250 μl serum free medium and added into the insert in theupper chamber of the transwell. The cells were then incubated at 37° C.with 5% CO₂ for 16 hours.

The insert was then removed from the well and the non-migrating cellsfrom the upper side of the membrane were removed gently using a cottonbud. The lower side of the membrane was fixed with methanol followed byDiff Quik solution I and II (LabAids) for 6 minutes each. Ten randomlyselected microscope fields at 200× magnifications under the opticalmicroscope (Nikon) were taken. Results were expressed as the averagenumber of migrated cells at each condition normalized to the averagenumber of migrated cells under 2D culture, in triplicates.Dose-dependent migratory responses of hMSCs to different dosage ofFractalkine (0-80 ng/ml) and SDF-1β (0-400 ng/ml) were studied todetermine the sub-optimal concentrations of these chemoattractantsbefore comparing the migratory activities of different MSCsubpopulations.

Results

The migratory response of hMSCs is chemokine dose-dependent as shown inFIGS. 2A and 2B for both Fractalkine and SDF-1. FIGS. 3A-3C show thesignificantly higher migratory activities as shown by the normalizedmigratory activities in the 3D migrated subpopulation than othersubpopulations including the 3D cultured and the 3D remained. FIG. 4shows the dose-dependent increase in the migratory activities as thecollagen barrier concentration increases, indicating that the higher thecollagen barrier capacity, the better the migratory activities of theselected population. Therefore, a gradient collagen barrier may selectand enrich some MSC subpopulations with super migratory capability.

The temporal morphological change of the cell-matrix microspheres withdifferent cell densities and collagen matrix densities were recorded.

Microspheres at day 0 showed individual cells embedding in the collagenmatrices and the microspheres were still transparent. Microspheres athigher cell densities such as 1×10⁵ and 5×10⁵ cells/ml and lowercollagen matrix densities of 0.5, 1.0 and 2.0 mg/ml contract as timegoes by and become more opaque and dense. This indicates that hMSCs arereorganizing the matrix to form a tighter matrix in the microspheres.Microspheres at lower cell density (2×10⁴ cells/ml) took much more timeto contract to a constant size while microspheres with higher collagenmatrix density, 3.0 mg/ml, showed so little contraction that the matrixappears transparent. The extent of hMSC-induced collagen microspherescontraction was directly proportional to the cell density, collagenconcentration and droplet volume, establishing that that theseparameters can be used to control the final size of the microspheres.The hMSC-collagen microspheres, after reaching the equilibrium, can bemechanically manipulated by forceps and are resistant to the shearstress and turbulence generated during pipetting up and down at rapidrate such as 20 ml/min or even vortexed with maximal speed. As a result,these microspheres are mechanically stable enough to resist shear stressgenerated during microsyringe injection and are ready for injection andimplantation for cell therapy and tissue engineering purposes.

The in vitro migratory activities of the functionally selected (3Dmigrated) MSCs derived from human adipose tissue were consistent withthat from human bone marrow (FIGS. 6A-6C). Functional selection is alsocell density dependent. As the cell density increases, the functionallyselected cells have better migratory activities (FIG. 7).

In addition, functionally selected MSCs secrete more matrixmetalloprotease 1 (MMP1), i.e. collagenase, than other control groups.This further suggests that these functionally selected cells havefunctional difference, in this case, the ability to digest collagenmatrix, comparing with other groups.

Example 3 Engraftment of 3D Migrated Cells in Hepatectomized NOD/SCIDMice

Materials and Methods

8 week old, 25 gram NOD/SCID mice were anesthetized and then the median,left and caudate lobes of the liver, as well as the gall bladder, wereremoved, leaving the right lobe of liver. Two to three hours wereallowed for the mice to recover after the surgery. After that, the micewere anesthetized again. Cell injection was done via the tail vein. Twomillion migrated cells suspended in 100 μl 1×PBS were used for cellinjection.

At 48 hours 1 week and 1 month, mice were sacrificed and the livercollected for human cell marker analysis using flow cytometry andimmunohistochemistry. For flow cytometry, cells from the harvested liverwere isolated by incubating with collagenase for 20 minutes followed byfiltering through the cellular sieve (BD Biosciences). The cellsuspension was centrifuged and the supernatant was removed. Blood cellsin the cell suspension were lysed by incubating with ACK buffer for 5minutes. After that, the lysis reaction was stopped by topping up with1×PBS and centrifuged. Supernatant was removed and the cells wereresuspended in 1×PBA. Cells were stained with phycoerythrin (PE) orfluorescein isothiocyanate (FITC)-conjugated mouse monoclonal antibodies(mAbs) according to the instructions provided by the manufacturers. Thefollowing mAbs were used in this study: anti-human HLA-ABC (BDPharmingen™, cat. #555552) and anti-human CD73 (BD Pharmingen™, cat.#550257). The following isotype controls were used in this study:PE-mouse IgG_(1κ), (BD Pharmingen™, cat. #555749) and FITC-mouseIgG_(1κ), (BD Pharmingen™, cat. #555748). Flow cytometry was done byEPICS Elite ESP high performance cell sorter (Coulter Electronics) anddata was analyzed using WinMDI 2.9 software. For immunohistochemistry,samples were fixed in 4% paraformaldehyde at 4° C. overnight. The fixedlivers were washed with 1×PBS and dehydrated in increasingconcentrations of ethanol ranging from 50% to 100%, then wax embeddingand paraffin sectioned (7 μm). Immunohistochemistry of human markers wasconducted to determine if there were any human cells in the livers. Thesamples were blocked by incubating with 10% normal horse serum (NHS)diluted in 1×PBS for 45 minutes at room temperature. The samples werethen incubated with primary antibody, beta-2-microglobulin (Santa Cruz,cat. #sc13565) at 1:100 dilution at 4° C. overnight. The samples wereincubated with 3% H₂O₂ in methanol at room temperature for 30 minutes toblock the endogenous peroxidase. The samples were washed with 1×PBStwice followed by incubating with horse anti-mouse secondary antibody at1:200 dilution at room temperature for 30 minutes. The samples werewashed with 1×PBS twice and were incubated with reagent ABC at roomtemperature for 30 minutes. After that, the samples were washed with1×PBS twice and were incubated with DAB substrate at room temperaturefor 5 minutes. Finally, the samples were washed with 1×PBS twice andwere counterstained with hematoxylin. The samples underwent dehydrationusing ethanol ranged from 70% to 100% and then dewaxed using xylene.They were mounted with permanent mounting medium (Depex) with coverslip(Marienfeld).

Results

Flow cytometry analysis showed that the functionally selected hMSCs (3Dmigrated cells) showed significantly better in vivo engraftment rate inthe hepatectomized liver of the NOD/SCID mice particularly at later timepoint at 1 month as compared with the unprocessed hMSCs (2D culturedcells). Moreover, analysis of the sections showed the engraftment of the3D migrated hMSCs at 48 hours post-injection based on the immunopositivestaining.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of skill in the artto which the disclosed invention belongs. Publications cited herein andthe materials for which they are cited are specifically incorporated byreference.

Those skilled in the art will recognize, or be able to ascertain usingno more than routine experimentation, many equivalents to the specificembodiments of the invention described herein. Such equivalents areintended to be encompassed by the following claims.

1. A method to functionally select a subpopulation of cells withimproved engraftment rate from a crude preparation of stem cellscomprising selecting the stem cells from the crude preparation of stemcells that migrate through a barrier or gradient of barrier material inresponse to a chemoattractant, wherein the stem cells that migratethrough the barrier or gradient of barrier material have an improvedengraftment rate compared to the stem cells failing to migrate throughthe barrier or gradient of barrier material.
 2. The method of claim 1,wherein the crude preparation of stem cells comprise multipotent orpluripotent cells.
 3. The method of claim 2, wherein the crudepreparation of stem cells comprise stem cells obtained from bone marrow,adipose tissue, umbilical cord blood.
 4. The method of claim 3, whereinthe crude preparation of stem cells are obtained by adhesion selectionor flow cytometry sorting.
 5. The method of claim 2, wherein the crudepreparation of stem cells are selected from the group consisting ofinduced pluripotent stem (iPS) cells, embryonic stem (ES) cells,mesenchymal stem cells, or undifferentiated cells derived from theculture of iPS cells, ES cells, mesenchymal stem cells, and acombination thereof.
 6. The method of claim 1, wherein the barrier orgradient of barrier material comprises a biomaterial.
 7. The method ofclaim 6, wherein the biomaterial is selected from the group consistingof collagen, fibrin, extracellular matrix materials, self-assembledpeptides, hyaluronic acid, and combinations thereof.
 8. The method ofclaim 1, wherein the barrier is a homogenous barrier comprising ahomogenous fiber density or barrier capacity, or wherein the barrier isa gradient barrier comprising an increasing fiber density or barriercapacity.
 9. The method of claim 1, wherein the barrier is in the formof a microsphere.
 10. The method of claim 1, wherein the barrier isselected from the group consisting of a block gel, patch, or thin film.11. The method of claim 1, further comprising culturing cells entrappedin the barrier or gradient of barrier material in suspension or freefloating cultures prior to selection to allow the cells to adapt to theenvironment barrier and the barrier to contract to an equilibriumstatus.
 12. The method of claim 11, comprising culturing the entrappedcells for about 1 to about 14 days.
 13. The method of claim 11,comprising culturing the entrapped cells for about four days.
 14. Themethod of claim 13, comprising transferring the entrapped cells into atissue culture container comprising an adherent surface in the presenceof a medium containing a chemoattractant, allowing the entrapped cellsto attach to the adherent surface, culturing cells migrated out from thebarrier for 1 day to 14 days, and selecting the cells migrating throughthe barrier or gradient, wherein cells that migrate through the barrieror gradient have improved engraftment rate.
 15. The method of claim 14,wherein the medium comprises serum.
 16. The method of claim 14, whereinthe cells are selected by detaching the cells from the adherent surface.17. The method of claim 1, wherein the selected cells are enrichedsubpopulations of MSCs with enhanced spontaneous migratory activitiesand directional migration towards chemokines secreted from thecirculation during tissue injuries.
 18. A method of treating tissuedamage in a subject comprising administering the cells selectedaccording to the method of claim 1 to a site of tissue damage in thesubject.
 19. The method of claim 18, wherein the subject is diagnosedwith a myocardial infarction, osteogenic defect, or liver disease. 20.An isolated subpopulation of cells obtained by the methods of claim 1.21. A kit comprising the subpopulation of cells according to claim 20,and a means for administering the cells.
 22. The kit of claim 21,further comprising a device for systemic injection or implantation ordelivery of the selected cells.